Complementary metal-oxide-semiconductor (“CMOS”) image sensors (“CIS”) may generate inaccurate image data due to dark current in the pixels themselves and variation in the level of dark current from pixel to pixel. Each pixel of a CIS array provides an output voltage that varies as a function of the light incident on the pixel. Unfortunately, dark currents add to the output voltages and degrade the picture provided by the imaging system. To generate accurate image data, it is desirable to estimate dark current and level correct for it.
Most image sensors require some form of calibration before use so that the data obtained from the image sensor can be used to produce digital images that faithfully reproduce the optical characteristics (intensity, color, etc.) of the captured scene or object. Some calibrations can be carried out once and remain valid for every subsequent use of the image sensor, but other calibrations must be carried out for every single use of the image sensor. Black level calibration is one of the calibrations usually performed for every single use of an image sensor. As its name implies, the purpose of a black level calibration is to determine the black level of the image sensor. The black level calibration effectively sets a threshold below which digital data values obtained from the image sensor will be considered to represent the color black, or in other words, represent the absence or substantial absence of light. The threshold value is then used to adjust the values obtained from other pixels in the array. Accurate black-level calibration helps to achieve a digital picture with full contrast and subtle details in dark shadow regions. If the black level is too low, information in dark areas may be lost; if the black level is too high, signal range may be sacrificed.
Black level calibration is traditionally done with frame-wise subtraction of the current frame to a global dark row signal. This method removes dark current as well as the readout channel offset leaving just the image data. However, in situations where the dark current is not uniform across the pixel array, this method is less effective in obtaining black level calibration.
In frame exposure mode, the shutter and integration of a pixel array is done simultaneously across the entire pixel array. However, readout is done one row at a time, so there is an integration time difference from the top to the bottom of the pixel array. Non-uniformity of the dark current in frame exposure mode may cause vertical shading. Other causes of vertical shading include temperature gradients, process gradients, and pixel output settling.
Conventional black level calibration techniques are executed in the digital domain. One drawback of digital black level correction is that the image data loses its dynamic range after the digital subtraction operation.